In processing applications involving the deposition of gases, various films may be formed along the surface of a workpiece exposed to a gas. An example of such a workpiece is an integrated circuit wafer. A film formed on a wafer is typically formed by distributing a gas into a chamber holding the wafer. An inlet tube provides a laminar flow of gas to the chamber. Once the gas enters the chamber, it is desirable to maintain a laminar flow within the chamber so that the gas is evenly distributed to the wafer.
In the prior art, numerous efforts have been made to achieve a uniform gas distribution inside the chamber in order to produce a uniform deposition of gases across the wafer. Unfortunately, these efforts have commonly resulted in gas flow within the chamber having areas of nonuniform flow known as vortices. Each vortex within the flow of gases may cause a nonuniform distribution of gas to the wafer. As a result, a nonuniform film may be formed across the wafer. This nonuniform film has detrimental effects in that integrated circuits subsequently produced on the wafer may not function properly, if at all.
The prior art has heretofore included at least three different mechanisms used in an effort to achieve a uniform gas distribution inside the chamber. One example of a prior effort to provide uniform distribution includes the use of a baffle or flat plate placed in line with the inlet tube of the chamber. The baffle disrupts the gas flow to the center of the wafer and causes complex flow fields with turbulent-like vortices. Certain baffle configurations may produce flow fields that distribute gases relatively uniformly over the wafer and result in noncentrally distributed films. However, determination of such configurations is empirical and varies from reactor to reactor. Additionally, the complex flow fields may cause the system to be relatively unstable under varying operating conditions. Further, the gas distribution is poorly understood and controlled and the system may be difficult to scale to other geometries.
A second example of a prior art effort to provide uniform gas distribution within the wafer chamber is a shower head configuration. In this apparatus, the reactant gases are injected onto a plate with an array of pin holes which distributes the gases to the wafer. These systems have complex flow fields, and depending on the size of the holes, may cause the formation of jets which impinge the gases on the wafer. Thus, these systems are subject to drawbacks similar to the baffle design discussed above.
A final example of a prior art effort to obtain uniform gas distribution within the wafer chamber is the use of a dispersal ring. The dispersal ring is displaced around the inlet tube and introduces one of the reactant gases noncentrally. As a result, a noncentrally distributed film is formed with improved uniformity. However, the resulting flow fields induce a large vortex about the dispersal ring. As a result, a majority of the added reactant bypasses the wafer and is wasted. An additional drawback of the dispersal ring is that reactants are mixed in close proximity to the wafer. This yields low deposition rates in systems where homogeneous reactions are necessary to produce reactive precursors. Thus, the dispersal ring gives rise to inefficient use of the reactant gases. Additionally, the flow resulting from the dispersal ring yields problems similar to that of the baffle and shower head systems discussed above.
Therefore, a need has arisen for a method and apparatus for distributing gas to a workpiece such as a wafer or the like in a laminar flow in order to provide a uniform deposition of the gas across the entire workpiece.